The invention relates generally to devices for charging, formatting, and reconditioning batteries. More particularly, the invention relates to devices for rapidly charging and/or reconditioning a battery that have flexibility in the type of charging profiles supported. These charging profiles primarily consist of charging stages which include repeating charge pulses, discharge pulses, such as by applying a load across the battery, and wait or rest periods. An example of an apparatus for rapidly charging and reconditioning a battery is disclosed in U.S. Pat. No. 5,998,968, which is assigned to the assignee of the present invention and expressly incorporated herein by reference.
Batteries are commonly used to provide a direct-current (dc) source of electrical energy in a wide variety of applications. A battery generally consists of a plurality of cells grouped together in a common container and electrically connected to provide a particular dc source. For example, four 1.5 volt cells rated at 1 ampere (amp) may be series connected to provide a 6 volt dc source rated at 1 amp. Cells may also be connected in parallel, e.g., four 1.5 volt cells rated at 1 amp connected in parallel provide a 1.5 volt dc source rated at 4 amps, or in a combined series-parallel fashion. The cell consists of two electrodes, one connected to a positive terminal and the other connected to a negative terminal, which serve as conductors through which current enters and leaves the battery. The electrodes or plates are surrounded by an electrolyte that acts, electrochemically, upon the electrodes in a manner dependent upon the nature of the materials used to comprise the battery.
The nature of the materials used to comprise the battery will also determine if the electrochemical reaction is reversible or not. Cells with reversible reactions are considered of "secondary" type, whereas non-reversible reactions are considered of "primary" type. The present invention is essentially concerned only with secondary cell type batteries, which may be recharged by forcing an electrical current through the battery in a direction opposite to that of discharge, thereby reversing the electrochemical reaction. The storage capacity of a battery is generally rated according to ampere-hour capacity, e.g. a battery that delivers an average of 10 amps without interruption over a 2 hour period, at the end of which time the battery is completely discharged upon reaching a low voltage limit, has a capacity rating of 20 (10.times.2) amp-hours. If discharged at a faster rate, e.g. over one hour instead of two, the battery will deliver less than the rated capacity.
Battery charging is, in its simplest terms, accomplished through delivery of a current to a battery, thereby ionizing the plates to opposing potentials (voltages or electrical pressures) and reversing the electrochemical process that occurs when the battery is used to supply energy to a load. With a linear charger, this is achieved through the use of a marginally higher charger voltage vs. what the battery's maximum rest voltage is. Usually, in the most rudimentary charger types, minimal consideration is given to regulation, whether it be current or voltage. The battery is simply allowed to drift up to its maximum potential. This is useable, albeit slow, and also does not treat the battery equally over time. When the battery is lowest, the most amount of current flows. If the time that it takes for the battery to charge is divided into five segments as shown in FIG. 8, often, the first time segment would appear to have charged the battery by about 70%, the next segment would be up to 90%. The remaining three time segments gradually approach the 100% charge level.
When charging a battery, it is preferred to use a charge voltage that is only marginally higher than the full battery potential. This is because higher voltages will increase the amount of secondary reactions that take place. An example of a secondary reaction would be the electrolysis of water in the electrolyte. The water will be split into oxygen and hydrogen gases. Another effect of higher voltages is waste heat. Energies not stored or used for secondary reactions are converted into heat. More advanced chargers use a pulsing technique to send bursts of energy to the battery at higher voltages. This is done in the hopes of reversing the secondary reactions while they are still within range of the plates. While this does get the job done quicker, the effects are to gradually destroy the battery by heating, and eventually evaporating, the electrolyte inside. One unfortunate effect of pulse charging is that in order to deliver the same amount of current over the charge duration, the pulses must be of higher current than a non-pulsing charger would be. This causes an increase in the "Joule effect:" losses that occur because of the internal resistance of the battery. For example, 10 amps delivered continuously into a battery that has a resistance of 10 milliohms results in 1 watt of wasted energy (P.sub.w =I.sub.CHG.sup.2.times.R.sub.BATT). A hypothetical pulsing charger with a duty cycle of 50% would have to deliver 20 amps in order to deliver the same level of current over the charge duration. The Joule effect losses quadruple to 4 watts during the pulses, yielding an average loss of 2 watts (20 A.sup.2.times.10 milliohms.times.50%). While pulse type charging has benefit, it may not be desirable over the full duration of charge and it may be advantageous to vary the parameters of the pulsing at different stages of the recharging process.
An alternative path is to use a constant current source, assuming that the battery is capable of accepting a set amount of current at any time. These chargers regulate current by automatically adjusting the voltage so a predetermined amount of current is delivered. Volume of current is the design factor, not the voltage of the battery. Unfortunately, the results here can be less than desirable. If the amount acceptable to the battery is overestimated, the battery becomes less willing to accept the current. The charger instantly compensates by raising the charge voltage. The battery heats up, which further exacerbates the situation by causing the battery to become even less able to accept further charging. The cycle continues. Without limits, this scenario could escalate to destruction of the battery.
The magnitude of the charge or load, the duration of the charge, load, and rest stages, and the particular sequencing of the three elements is dependent upon the particular battery type being charged or reconditioned. Further, the sequencing, magnitude, and duration of these stages may be varied during the process of charging or reconditioning the battery. Ideally, the batteries are measured while under load to determine the level of charge as this tends to give a more accurate measurement of the battery that would otherwise be colored by the battery's impedance, secondary reactions and the charge delivered.
Conventional battery chargers merely deliver steady voltages or constant current. A problem with the steady voltage chargers is that the battery cannot absorb all of the energy delivered in the early stages of charging and the charging process takes an excessive amount of time to complete due to the low current that flows in the later stages of charging. A problem with the constant current is that to completely charge the battery using a current to facilitate rapid charging, the battery heats up during the last part of the charging since there is more energy delivered than the batter can absorb at this time. Whenever there is more power being delivered to the battery than it can absorb, the excess is spent via electrolysis of the medium or conversion to heat energy.
Some chargers include a few charging stages that are sequenced through to charge the battery. However, these stages are hardware driven and require a hardware change to charge a battery with different charging characteristics.
One problem associated with prior art battery chargers, such as so-called wall-wart type chargers, concerns the amount of time required to fully charge a battery. Such known chargers generally require a period of 12-24 hours of continuous charging to fully charge a typical battery. This results in undesirable, extended down-times for devices being served by the battery being charged or the expense of purchasing and maintaining multiple batteries.
Another problem associated with the prior art is that these chargers can control only either the power supply voltage or current to charge the battery. The most efficient method of charging a particular battery at a particular state of charging may require a constant current, constant voltage, or limiting of both current and voltage.
Another problem associated with the prior art is that these chargers require hardware changes to charge batteries during different charging stages.